We have in recent years witnessed digital information being offered in dramatically increasing quantities, as various contents including video and audio have become available in digital format. In response to this trend, development is under way to design optical disk devices suitable to increasing capacity and density. Meanwhile, considering that the quality of reproduction signals deteriorates with increase in density, various solutions are suggested to improve the quality of reproduction signals.
Reproduction signals carry, in addition to image and other main information, servo information and other various kinds of control and management information. Among those kinds of information, clock information is one of the most important, because it serves as a reference for operations of all the circuits that record or reproduce information. Japanese Laid-Open Patent Application No. 11-16216/1999 (Tokukaihei 11-16216; published on Jan. 22, 1999) discloses an optical disk and an optical disk device that are capable of performing error-free readout with improved clock information quality.
The above conventional optical disk and optical disk device will be further explained in the following.
First, the format of the optical disk will be explained. FIG. 6(a) shows an arrangement of a sector. Each sector is divided into 46 segments. Each segment serves either as an address segment or as a data segment. Here, AS0 located at the head of the sector serves as an address segment, while DS0 to DS44 as data segments.
FIG. 6(b) shows an arrangement of an address segment. The address segment includes a clock mark field (CM field), an address field, a preamble field, and other fields. In each field, a predetermined signal is recorded in advance in the form of a variation in the shape of a pit or groove.
The clock mark field records in advance a clock mark from which a clock signal is generated as mentioned in the foregoing. The address field records the address of the sector. The other fields are provided as necessary so as to control address readout or ensure a readout margin.
FIG. 6(c) shows an arrangement of a data segment. The data segment includes a clock mark field (CM field) at its head as does the address segment. The other field serves as a data field, where main information is recorded and reproduced by a magneto-optical recording technique. Each segment is 63.5 byte long, for example. Clock marks are then found at identical intervals equaling 63.5 bytes.
Next, the clock mark will be explained. FIG. 7 shows a clock mark on the disk. The clock mark is formed in the form of a convexity in the groove and of a concavity in the land as shown in FIG. 7, causing a fluctuation in the quantity of light with a move of a laser spot along the tangent of the track. The clock mark is detected, using a photodetector divided perpendicular to the tangent of the track into two parts, as a differential signal, i.e., a tangential push-pull signal (TPP signal), supplied by those two parts of the photodetector. FIG. 7 shows how the TPP signal fluctuates. Cyclic clock signals are detected by binarizing the TPP signal using, for example, a zero-cross comparator.
The aforementioned detection of a clock signal from the clock mark allows the shape and other parameters of the clock mark to be specificized independently from the main information recorded by a magneto-optical recording technique. In addition, the use of TPP signals better prevents the conditions under which tracking is controlled from negatively affecting the detection of a clock signal than the use of push-bull signals. These factors contribute improvement of the quality of signals. Therefore, a clock can be reproduced with a relatively short mark length, and opens up a possibility to further increase the recording density, compared to the clock information recorded in wobbles whereby the side walls of a track groove are formed with wobbles.
Now, the conventional optical disk device will be explained. FIG. 8 shows a diagram representing an arrangement of major components of a recording and reproduction signal processing section for use with an optical disk. The optical disk 1 is rotated by a spindle motor 2. The optical disk 1 may be of any type; here, the explanation will be made based on the assumption that the optical disk 1 is a magneto-optical disk.
The optical disk 1 is irradiated on its bottom surface with a light beam projected through an objective lens 3 disposed on a pickup 4. The intensity of the light beam is suitably controlled by the LD drive circuit 6, and hence differs between recording and reproduction operations. The light reflected at the optical disk 1 is detected by a photodetector provided inside the pickup 4. The reflection is separated into a TPP signal, an RF signal, and a servo signal (not shown).
Bit clocks are generated from the TPP signal by a clock generation circuit 13. The foregoing description tells that only one clock signal is detected from each segment, because each segment contains only one clock mark; however, bit clocks are generated in plurality with a suitably increased ratio by means of a built-in PLL circuit in the clock generation circuit 13. Here, since each segment is specified to include 63.5 bytes of data, 508 (63.5 bytes×8 bits) bit clocks are generated per segment.
The generated bit clocks are supplied to data processing and other circuits, including a demodulation circuit 14, a modulation circuit 8, an A/D converter 11, a reproduction data processing circuit 15, and a recording data processing circuit 10 as necessary. The RF signal is sampled by the A/D converter 11, and then demodulated by the demodulator circuit 14.
The signal demodulated by the demodulator circuit 14 is transmitted to the reproduction data processing circuit 15, where the demodulated signal is processed so as to recover data therefrom. Errors found in the data recovered in the reproduction data processing circuit 15 are corrected in an error correction circuit 17.
For recording, the data with an additional error correction code provided in the error correction circuit 17 is sent to the recording data processing circuit 10, where the received data is divided into sets of data to be recorded in respective segments, so as to generate sector data. Each set of data is then modulated by the modulation circuit 8 into predetermined modulated signals. The modulated signals are translated into a magnetic field by a magnetic head drive circuit 7 and a magnetic head 5. The magnetic field in turn records information on the optical disk 1 in collaboration with a light beam emitted by the pickup 4.
Next, the conventional optical disk device will be explained in relation to its recording and reproduction operation.
First, the explanation will focus on the recording and reproduction operation performed by the conventional optical disk device under normal conditions, that is, when the clock mark is not defective.
For recording, the address recorded in advance in the address segment located at the head of the sector is all reproduced so as to confirm that the reproduced address matches the target address. If so confirmed, the data with an additional error correction code provided in the error correction circuit 17 is recorded in data segments.
For reproduction, the address segment located at the head of the sector is reproduced to confirm that the reproduced address matches the target address. If so confirmed, the data segment is reproduced. Only the data is recovered and transmitted to the error correction circuit 17, where error correction is performed in predetermined procedures. The conventional optical disk device thus performs recording and reproduction operation.
The recording and reproduction operation mentioned in the foregoing can be performed in a stable manner regardless of the quality of signals reproduced from a data field, because of the use of bit clocks generated from high-quality clock signals.
Incidentally, the detection of clock signals by means of clock marks boasts excellent performance. The method, however, is susceptible to disk flaws, because the use of clock marks enables the detection of clock signals with shorter mark lengths, but inevitably increases, with decreasing mark lengths, the likelihood of the presence of even a small flaw negatively affecting the clock mark. A normal clock signal cannot be detected from a clock mark, if the clock mark is defective due to a flaw.
Now, the explanation will focus on the recording operation performed by the conventional optical disk device with the optical disk 1 when a clock mark is defective due to a flaw.
FIG. 9 shows, in its top half, the position of data recorded on the optical disk 1 under normal conditions. The sets of data recorded in the respective data segments DS0 to DS44 are identical in size to each other, while the bit clocks in those data segments are also identical in number at 508 to each other; therefore, the data segments have equal lengths.
FIG. 9 shows, in its bottom half, the position of data recorded on the optical disk 1 when a clock mark is defective. Here, an example is taken where the clock mark located at the head of the data segment DS3 is defective. The defect of the clock mark disrupts normal generation of bit clocks in the clock generation circuit 13, resulting in the generation of bit clocks with a higher or lower frequency than that of the standard bit clocks. Whether the frequency of the bit clocks increases or decrease over that of the standard bit clocks is determined by variations in the frequency and other conditions of the bit clocks of the data segment DS3 and its preceding data segments. Here, the explanation will focus on bit clocks with a decreased frequency.
Since the frequency of the bit clocks decreases, the data is recorded in the data segment DS3 with extended mark lengths in comparison to standard mark lengths. The data, which would normally be recorded in its entirety in the data segment DS3, does not fit into the data segment DS3 with its tail spilling over into the head, i.e., the clock mark field, of the data segment DS4. Let us assume that the number of bits spilled over equals N.
The aforementioned explanation may be rephrased with a term, “the number of bit clocks in a data segment.” The standard number of bit clocks in a data segment equals 508 as mentioned in the foregoing. However, the defect in the clock mark located at the head of the data segment DS3 lowers the frequency, that is, extends the cycle, of the bit clocks in the data segment DS3 and thereby reduces the number of the bit clocks to 507 or even further. Here, it is assumed that the number of bit clocks in the data segment DS3 equals 508-N.
Meanwhile, the clock mark located at the head of the data segment DS4 restores the frequency of the bit clocks to a standard-frequency, enabling the data after that to be recorded with normal mark lengths. However, since the data segment DS3 is extended by N bits in recording, the sets of data recorded in the data segments subsequent to the data segment DS3 are displaced by N bits. Hence, the last N bits of each set of data that should have been recorded in the data field of one of the data segments DS3 to DS44 are displaced and recorded in the clock mark field of the subsequent data segment. The displacement is corrected when the address segment located at the head of the subsequent sector is reproduced. Beginning from the first data segment of the subsequent sector, the data is recorded without being displaced.
Now, the explanation will focus on the reproduction operation performed by the conventional optical disk device when the data includes some bits that are displaced in a recording operation. The reproduction data processing circuit 15 includes a built-in buffer memory for temporarily recording data. FIG. 10 shows, in its top half, the position of data recorded in the buffer memory under normal conditions, and, in its bottom half, the position of data recorded in the buffer memory when the clock mark is defective.
The arrangement of reproduction data in the buffer memory is determined by bit clocks from the clock generation circuit 13. The defect is present only in the clock mark field of the data segment DS3. Therefore, up to the data segment DS2, the data is not at all affected by the defect: the data can be reproduced correctly and arranged in standard position At the head of the data segment DS3 where the clock mark is defective due to a flaw, clocks are not generated normally in a data reproduction operation. Hence, the frequency of the clocks is increased or decreased over the standard frequency.
Supposing that the frequency of the bit clocks deviates identically in recording and reproduction operations, the data in the data segment DS3 can be reproduced normally except the last N bits recorded in the clock mark field of the data segment DS4.
However, typically, conditions differ between recording and reproduction operations, and hence the frequency does not deviate identically. When this is the case, the number of bit clocks in the data segment DS3 in a reproduction operation differs from that in a recording operation, obstructing normal reproduction. Let us assume here that the difference of bit clocks in number between recording and reproduction operations corresponds to M bits.
At the head of the data segment DS4 where another clock mark is located, the frequency of the bit clocks is restored to a standard, enabling error-free reproduction of data in the data segment DS4 and its subsequent data segments. Note that the aforementioned displaced N bits cannot be reproduced, because they have failed to be recorded in a standard segment range, spilling over into the head of the subsequent data segment. In other words, in each of the data segments DS4 to DS44, the last N bits of data are lost, whereas the rest of the data is reproduced normally.
The data is reproduced correctly on a bit-by-bit basis. However, if the number of clocks corresponding to the data segment DS3 differs between recording and reproduction operations, the data in the data segment DS4 and its subsequent data segments is not divided correctly into 1-byte pieces of data. Typically, data reproduction, error correction, and other operations are performed on a byte-by-byte basis; if the data is not correctly divided into 1-byte sets of data, the data cannot be reproduced correctly.
As mentioned in the foregoing, in the data segment DS3, there exists a difference in the number of bit clocks between recording and reproduction operations, corresponding to M bits. Therefore, the data in the data segment DS4 and its subsequent data segments are displaced by M bits, causing none of that data to be reproduced correctly. FIG. 10 shows the data being displaced by M bits off a standard data position.
In short, if, the conventional optical disk and optical disk device fails to correctly detect the clock signal, the data-is displaced and cannot be recorded and reproduced.